In automotive vehicles, the rotational power (motive force) generated by various types of engines such as internal combustion engines, motors and the like is transmitted to a hub from a differential gear via plural transmission shafts of a half shaft, a spline shaft, etc, thereby tires thereof rotate.
The differential gear and the spline shaft are connected to each other through a so-called inboard side constant velocity joint, whereas the spline shaft and hub are connected to each other through a so-called outboard side constant velocity universal joint. Generally, the inboard side constant velocity universal joint performs a function to ease both an angular displacement and an axially directed displacement of the spline shaft, and the outboard side constant velocity universal joint carries out a role of easing the angular displacement of the spline shaft. As a result thereof, transmission of drive forces that accompany changes in an advancing direction, and more specifically changes in angle, are carried out rapidly, while upward and downward movements of the suspension are absorbed. As the inboard side constant velocity universal joint, a sliding type of constant velocity universal joint is employed, which is capable of making displacements in an axial direction of the spline shaft.
Recently, various ongoing research and development has been conducted with respect to inboard side constant velocity universal joints, which even more rapidly respond to axially directed displacements of the spline shaft. When this type of inboard side constant velocity universal joint is used, because the inboard side constant velocity universal joint is separated relatively from the engine, it becomes difficult to receive radiated heat from the engine. Due to this fact, it has been attempted to cover the connecting portion between the inboard side constant velocity universal joint and the spline shaft using a resin-made boot, which conventionally cannot be used out of fears concerning heat resistance.
Formerly, as materials used for these types of boots, chloroprene rubber (CR) and chlorinated polyethylene rubber (CM) compositions have been used, wherein in addition, grease is enclosed inside of the boots. As for the greases used, greases such as those disclosed in Patent Document 1 and Patent Document 2 may be cited.
Patent Document 1: Japanese Patent Publication No. 8-23034
Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-11482